The present invention refers to a method of making identity cores for ultrasound seals, each core comprising a brazed stack of disks each presenting at least one notch, the identity being at least in part defined by the random angular position of the various notches in the stack of disks.
Such a core is described in the patent document EP 0 638 250 A.
These cores are intended for being incorporated in seals or markers for controlling and handling dangerous materials, especially radioactive materials, in order to allow their control and to contend with frauds.
According to the terms of the Nuclear Non-Proliferation Treaty, any transport or storage of radioactive materials should be subjected to a permanent control by international inspectors. In order to comply with their mission the inspectors should be able to seal recipients and to verify the identity and integrity of such seals.
In the above cited documents bolts are described which insure locking of a recipient and which are supplied with an identity core. By applying an ultrasound reader to the head of the bolt, the identity can be read, and simultaneously it can be verified if the bolt has been manipulated since the most recent prior verification.
It is known to make use of random phenomena for the definition of the identity in order to avoid any fraudulent tentative to copy a seal. Thus the document cited above proposes to make identity cores by stacking a certain number of notched disks in such a way that, after a convenient assembly of the stack by soldering, the identity is defined at least in part by the random position of the notches in the stack, that means by the cavities which after soldering generate ultrasound echoes. The identity is thus defined after soldering a random assembly of disks to become one block, the random nature being based on their individual fabrication.
These cores may be combined with an integrity rod having a zone of preferred breaking in case of an integrity violation. By applying an ultrasound reader to a bolt incorporating such a core, the ultrasound response which is registered corresponds on the one hand to the identity of this seal and on the other hand, through the integrity status of the rod, to the integrity status of the bolt.
The present invention concerns the method for making identity cores which are intended to be used in such seals or others. In fact, it has been noted that the individual manufacturing of the cores is complex due to the large number of required cores (after each opening of a recipient a new core is necessary) and due to the small size of the cores (12 mm in diameter, 5 mm in height) and especially due to the quality requirements. For example, an absolute plane surface of the disks must be ensured, the thickness of a disk being in the range of one millimeter.
The method according to the invention is defined by the attached claims.